This invention relates to heat treatment of shaped articles of amide-imide or amic acid polymers. More particularly, the invention relates to improvements in the heat treating of shaped articles of polyamide-imide and polyamic acids prepared from trimellitic acid or derivatives thereof and aromatic diamines whereby the physical properties, and particularly, the thermal shock resistance of such articles are significantly improved.
In the fabrication of shaped articles of thermoplastic polymers, molten polymer is commonly passed into a mold or through a die and then solidified. Shear stresses and some degree of orientation of the polymer chains develop during passage of the polymer into the mold or through the die, and when the polymer is cooled rapidly to below its glass-transition temperature, as in many commercial fabricating operations, these stresses become locked into the polymer chains. It is known that such internal stresses can be relaxed or removed by processes, commonly referred to as annealing or heat treating. As described in Encyclopedia of Polymer Science and Technology, Vol. 2 (1965) such processes commonly entail heating a shaped article at a temperature above the polymer's glass transition temperature or as close to the glass-transition temperature as can be attained without exceeding the deflection temperature of the polymer. Optimum heat treatment conditions vary from polymer to polymer, and even for a given polymer, optimum conditions can vary depending on part size and shape.
In the heat treatment of shaped articles of amide-imide and amic acid polymers, various chemical reactions such as imidization, chain extension, and crosslinking take place such that the molecular weights of the polymers are increased and the physical properties of the articles improved. However, such reactions also result in generation of water, carbon dioxide, and other gases during both shaping and annealing operations. At the high temperatures employed in annealing, these components vaporize and expand and, when entrapped within the articles, can exert pressures sufficient to deform the articles. Accordingly, provision must be made in the annealing of articles of polyamide-imides or polyamic acids for the removal of water and other gases contained and generated within the articles.
Prior art proposals to optimize the heat treatment of shaped articles of amide-imide and amic acid polymers have met with varying degrees of success. One such proposal which may be considered pertinent to this invention can be found in Amoco Chemicals Corporation Technical Information Bulletin TAT-5 (1975), wherein it is disclosed that the tensile strength, elongation, and deflection temperature of shaped articles of polyamide-imides identified by the Amoco Chemicals Corporation trademark TORLON can be improved by heating from 300.degree. F. to 500.degree. F. over a period of 48 to 192 hours depending upon the maximum thickness of the article. A related proposal is set forth in Amoco Chemicals Corporation Technical Information Bulletin TAT-5 (Rev.) (1977) wherein it is disclosed to preheat shaped TORLON articles having a maximum thickness of less than 5/8 inch from 220.degree. to 475.degree. F. over a period of one day with either continuous or stepwise increases in temperature. Alternately, the article can be preheated at 330.degree. F. for one day. Following preheating, the article is heated at 475.degree. F. for one day and then at 500.degree. F. for one day.
Heat treatment of shaped articles according to the above-described proposals results in improvements in physical properties such as increased tensile strength, elongation, and heat deflection temperature. Moreover, the articles are suitable for use at temperatures as high as 500.degree. F. so long as increases in temperature take place gradually. However, when articles heat treated according to the above-described proposals are subjected to large increases in temperature over short periods of time, deformation typically takes place, apparently due to a softening of the polymers and rapid expansion of water and gases incompletely removed during heat treatment and absorbed into the articles from the atmosphere. Exposure to large increases in temperature over short periods of time is referred to herein as "thermal shock." Resistance to distortion or deformation caused by thermal shock is referred to herein as "thermal shock resistance."
In view of the foregoing, it is apparent that a need still exists for a method for heat treating shaped articles of polyamide-imides and polyamic acids such that the articles are resistant to thermal shock. Accordingly, it is an object of this invention to provide such a method. It is a further object of this invention to provide an improved method for heat treating shaped articles of amide-imide and amic acid polymers whereby the tensile strength and elongation of the articles are improved. Other objects of this invention will be apparent to persons of skill in the art from the following description and appended claims.
It has now been found that the objects of this invention can be achieved by controllably heating shaped articles of polyamides-imides and polyamic acids such that a major portion of volatiles contained in the articles and generated therein are vaporized and removed during early stages of the heating and the polymer molecules undergo sufficient imidization, crosslinking, and chain extension to progressively increase the polymer's deflection temperature to substantially its maximum attainable value without deformation of the articles. The resultant heat treated articles are substantially free of water and other volatiles and the polymer molecules have undergone imidization, chain extension, and crosslinking to such an extent that only insubstantial amounts of moisture can be absorbed into the articles.